Usually, a method comprising sulfonating particles of a copolymer of styrene and divinylbenzene is known for the production of a cation exchange resin. Since a cation exchange resin having a high ion exchange capacity per unit weight is preferred, it is necessary to perform sulfonation uniformly throughout the entire copolymer particles from surface to inside. The styrene-divinylbenzene copolymer, however, has the defect that the inside of the particles does not easily undergo sulfonation.
In order to increase the ion exchange capacity of the styrene-divinylbenzene copolymer, a method which comprises sulfonating the copolymer particles while they are swollen with a swelling agent such as nitrobenzene, and a method which involves the use of a copolymer composed of styrene, divinylbenzene and acrylonitrile as the copolymer particles to be sulfonated have been proposed.
The former method can certainly give a resin having a high ion exchange capacity. Since, however, the swelling agent is used, the number of treating steps increases. Furthermore, the waste acid after sulfonation also contains the swelling agent and cannot be reused. Hence, this method is not economically advantageous.
The latter method is preferred in that no swelling agent is used. According to this method, the ion exchange capacity of the copolymer particles after sulfonation increases, but the copolymer particles undergo breakage and a large amount of cracked particles get mixed in the final product.